As the requirements for gas discharge laser systems, e.g., excimer and molecular fluorine gas discharge laser systems, e.g., used for semiconductor photolithography become ever more demanding, e.g., going to 0.12 pm FWHM and 0.25 E95% (E95) for next generation lithography tool usage, e.g., for attaining the desired critical dimensions and the like when exposing mask patterns onto a wafer bearing photoresist, it has become apparent that spectral symmetry changes could “fool” a laser bandwidth measuring apparatus (wavemeter) using, e.g., an empirical or semi-empirical bandwidth estimation model into predicting an incorrect E95 bandwidth, e.g., from the fringe widths of an etalon spectrometer, applied, e.g., to a linear array of photodiodes “PDA” for location and fringe intensity measurement and spectral width intensity measurements.
Applicant has developed a method and apparatus for dealing with this problem.